This invention relates to processes for treating a metal surface on which a protective coating has previously been formed and remains in place, with its protective qualities intact, on one part of the surface but is totally or partially absent from, or is present only in a damaged condition over, one or more other parts of the surface, so that its protective value in these areas of at least partial damage or absence has been diminished. (Usually the absence or damage of the initial protective coating has been unintentional and has occurred as a result of such events as imperfectly uniform formation of the initial protective coating, mechanical damage of the initial protective coating, spotty exposure of the initially coated surface to solvents for the initial protective coating, or the like.) Particularly if the surface in question is large and the damaged area(s) relatively small, it is often more economical to attempt to create or restore the full protective value of the original coating primarily in only the absent or damaged areas, without completely recoating the object. Such a process is generally known in the art, and will be briefly described herein, as "touching up" the surface in question. This invention is particularly well suited to touching up surfaces in which the original protective coating is a conversion coating initially formed on a primary metal surface, more particularly a primary metal surface consisting predominantly of iron, aluminum, and/or zinc.
A variety of materials have been taught in the prior art for the general purposes of the present invention, but most of if not all of them contain hexavalent chromium, which is environmentally undesirable.
One object of this invention is to avoid any substantial use of hexavalent chromium and other materials such as ferricyanide that have been identified as environmentally damaging. Other concurrent or alternative objects are to achieve at least as good protective qualities in the touched up areas as in those parts of the touched up surfaces where the initial protective coating is present and undamaged; to avoid any damage to the protective coating from contacting it with the touching up composition; and to provide an economical touching up process. Other objects will be apparent to those skilled in the art from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word "about" in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, "parts of", and ratio values are all by weight or mass; the term "paint" includes all similar materials that may be described by more specialized terms such as "varnish", "lacquer", "shellac", "base coat", "top coat", and the like; the term "polymer" includes "oligomer", "copolymer", "terpolymer", and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, or of generation in situ within a combination by one or more chemical reactions, as noted in the description, between other material(s) newly added to the combination and material(s) already present in the combination, and does not necessarily preclude other unspecified chemical interactions among the constituents of a mixture once mixed; specification of materials in ionic form implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole (any counterions thus implicitly specified should preferably be selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to one of the objects of the invention); any definition of an acronym or other type of abbreviation applies, without the need for repetition of the definition, to all subsequent uses of the same abbreviation and applies, mutatis mutandis, to grammatical variations of the original meaning and abbreviation; and the term "mole" and its grammatical variations may be applied to elemental, ionic, and any other chemical species defined by number and type of atoms present, as well as to compounds with well defined molecules.